1. Field
The present invention generally relates to coating materials, and more particularly to a coating material having low friction characteristics and heat and abrasion resistance, which is coated on a component (e.g., rotating shaft) adapted to operate in a medium temperature environment (400 to 500° C.) so as to minimize any friction and abrasion between such component and a bearing supporting the same. The present invention is further directed to a method of coating said coating material.
2. Background
Air bearings, which use hydrodynamic properties of air, are generally used to support a rotating shaft in a high-speed rotating apparatus. An example of such type of apparatus includes an air cycle machine (an essential part of an air conditioning system for aircrafts), a gas turbine, a turbo-charger, an auxiliary power device or the like. An air foil journal bearing is one example of such air bearings. The air foil journal bearing includes a bump foil and has inherent stiffness and damping property according to the shape of the bump foil.
FIG. 1 is a sectional view illustrating an assembled structure of a rotating shaft and an air foil journal bearing of prior art.
As shown in FIG. 1, the air foil journal bearing 1 includes: a housing 2; a shim foil 4 provided along the inner peripheral surface of the housing 2; a bump foil 6 located on the shim foil 4; and a top foil 8 provided on the bump foil 6 and being subjected to a load acting vertically with respect to the axial direction of the rotating shaft 9. The existing air blowers or compressors, which are operated at high speeds, are deficient in that cooling systems must be equipped thereto since the rotating shaft and the bearing are prone to damage due to a rapid temperature rise caused by a rubbing phenomenon. As one of the approaches for dealing with such a problem, the foils 4, 6 and 8 are typically made from the INCONEL material in order to enhance the durability and operation reliability at high temperatures. Particularly, the surface of the top foil 8, which is in direct contact with the rotating shaft 9, is coated with a hybrid type solid lubricant. Alternatively, a sol-gel coating or a coating material including molybdenum oxide (MoS2) may be applied to the top foil 8. Further, the abrasion resistance and lubrication may be enhanced by coating a coating material 9′ containing chromium oxide (Cr3O2) (e.g., “CORONA 910/920” disclosed in U.S. Patent Application Publication No. US2007/0003693A1 (Lee et al.)) on the surface of the rotating shaft 9 by plasma spraying.
The above-mentioned prior art coating materials have a low limited life for abrasion since they are applied thin on the surface of the top foil. The CORONA 910/920, which is coated on the rotating shaft, exhibits its ability only at a high temperature of greater than 750° C. Those materials cannot guarantee a good lubricating ability when applied to a turbomachinery such as a turbocharger, which is operated at a medium temperature of 400 to 500° C.
Accordingly, there is a need in the art to develop a coating material capable of withstanding friction, heat and abrasion, which occur due to contacting the bearing, when the rotating shaft is rotated at a high-speed without any oil supply in a medium temperature environment of 400 to 500° C. (particularly when the rotating shaft starts or stops). There is a further need in the art to develop a coating method that allows the durability of the coating material to be retained for a long time.